Connectives are the clearest structural tell
Vocabulary is easy to fix and easy to fake. Structure is harder. A generated draft tends to announce its own paragraphs: Additionally opens the second point, Moreover the third, Ultimately the conclusion. The words are correct and the paragraph is empty, because the connective is doing the job the argument should do.
Measuring it per paragraph rather than per document gives you something actionable. You are not told that the piece has eight connectives; you are shown which paragraphs start with one, and the fix is always the same: delete the opener and start on the subject.